There is a fundamental problem with Obama's plan and it's not about the engineers or the concept of private enterprise at all (which are the reasons Aldrin, Cameron and others endorse it), but the status of the private industry at present, which is not going to change for the foreseeable future.
Private enterprise is about money, of course, but today is about money now, and safe money. Low risk, quick profits, end of line. We're not in the '60s, '70s or '80s anymore. In the current climate, the computer revolution of the late '70s-early '80s would have never happened, just check out where Apple is now. Back in 1977 their product was the Apple ][: revolutionary, open, reasonably priced, something to tinker with and develop. Now it's the iPad: more of the same, locked, overprices, something to use as Steve Jobs wants you to use it and if you want to develop, you've got to pay. Had this philosophy been prevalent in 1977, they wouldn't have had the success they had because third party accessories and software that helped sell ][s wouldn't have ever existed.
The private sector is about immediate profits and reduced risks, and space stuff promises neither. If SpaceX can make money by simply launching satellites, that's where they'll stay. If any other company can make money by low-cost, unmanned stuff that's what they'll do. It's not in the hands of talented engineers or the rare visionary entrepreneurs, but in those of shareholders.
Unless the US Government intends to pursue the same method used in military contracts, that's not going to change and since NASA will not get the budget the DoD has, the writing is pretty much on the wall. Who's willing to design and build spacecraft if there's not a big, meaty contract there for the taking?
Maybe they should simply leave manned spaceflight to the USAF. They have the resources and the experience to deal with this kind of stuff, and the DoD budget to back them up.